The present invention relates to a device for controlling a steering force in a power steering apparatus provided with a hydraulic pressure reaction mechanism, and more specifically to a steering force control device for controlling the function of a power steering apparatus in response to the speed and steering angle of a vehicle.
It has been widely known, for example, from U.S. Pat. No. 4,034,825 entitled POWER ASSISTED VEHICLE STEERING issued on July 12, 1977 to Frederick John Adams, that a rotational torque from a steering wheel is increased by a power steering apparatus provided with a resilient torsion bar and then transmitted to a travelling wheel. In this patent, the operation of the power steering apparatus is controlled according to the speed of an automobile. More specifically, the aforesaid patent discloses that at the time of high speed drive, the operation of the power steering apparatus is weakened whereas at the time of low speed drive, the operation thereof is intensified. In the power steering apparatus as described above, for example, the rotation of the rotating shaft of the engine is transmitted to an oil pump by a pulley and an endless belt passed over the pulley, and oil within an oil tank is supplied by the oil pump to the power steering apparatus to strengthen the steering force. Furthermore, the rotation of a countershaft of a transmission of the vehicle is transmitted to a separate auxiliary oil pump, and oil from the oil tank is sucked into the auxiliary oil pump. A throttle valve is provided on a discharge port of the auxiliary oil pump. Oil having passed through the throttle valve is again returned to the oil tank, and pressurized oil is introduced from a middle portion between the discharge port of the auxiliary oil pump and the throttle valve into a hydraulic pressure reaction chamber for controlling the torsion of the torsion bar to control the operation of the power steering apparatus. More specifically, the auxiliary pump is driven to increase the number of revolutions thereof proportional to the vehicle speed as the countershaft of the transmission of the vehicle rotates rapidly, and the amount of discharge of the pump increases. Accordingly, at the time of high speed drive, high oil pressure is applied to the throttle valve, which results in application of the high pressure to the hydraulic pressure reaction chamber to weaken the operation of the power steering apparatus to render the operation of a steering wheel heavy. However, the dependence to the countershaft leads to a drawback that the amount of discharge of the auxiliary pump is too small to obtain a great hydraulic pressure reaction.
With respect to the characteristics of vehicle speed (V)--steering force (T), it has been assured from experiments that as shown in FIG. 7, at the low speed travelling of the vehicle, the steering force does not change so much; at the time of medium speed travelling of the vehicle, the force abruptly changes; at the high speed travelling of the vehicle, the force does not again change so much, which are preferable. It is not possible for a simple combination of a vehicle speed responsive pump and a fixed throttle valve as in the prior art to suitably obtain the desirable characteristic of vehicle speed (V)--steering force (T). Namely, it is impossible to suitably realize the characteristics of vehicle speed (V)--steering force (T) of various forms as shown in A, B and C of FIG. 9.
It has been further assured from experiments that even if the vehicle speed is the same, the value of the steering output (P) is varied according to a variation in steering angle as shown in the characteristic of steering angle (.alpha.)--steering force (T) of FIG. 8, whereby the safety and maneuverability of the vehicle may be further enhanced. FIG. 8(a) shows the ideal characteristic assured by the experiments and FIG. 8(b) shows the prior art characteristic. However, such an ideal characteristic cannot be expected as far as the well-known hydrualic pressure control is used.
Moreover, in the auxiliary oil pump driven by the transmission of the vehicle, at the time of low speed rotation, namely, at the time of low speed travel of the vehicle, there exists a problem that the amount of discharage of the oil pump is insufficient, and pulsation and pressure variation occur.
Furthermore, in the control of the prior art structure, the condition of the road surface cannot be fed back as information to the control of steering force, and therefore even if there is less friction in road surface such as snow roads, road surfaces at rainy days, etc., variation in steering force as required is not transmitted to the operator, and in addition, even if the road surface is uneven, it is not transmitted as variation in steering force to the operator.